Study On The Bending Performance And Bearing Capacity Calculation Method Of CFST Members With Lattice Angle

Concrete-filled steel tube(CFST)composite members with lattice angle steel and longitudinal stiffening ribs are a new type of CFST members.Compared with traditional concrete-filled steel tube members,this kind of member has better mechanical properties.In this paper,the bending performance and bearing capacity of this new type of concretefilled steel tube(CFST)members are studied by means of experimental research,nonlinear numerical simulation and theoretical analysis.The main contents are as follows:(1)Four groups of 8 bending members with different internal stiffening structures(no internal stiffeners,longitudinal stiffening ribs,lattice angle steel,longitudinal stiffening ribs and lattice angle steel)were studied.The results show that the concrete cracks in the tensile zone are delayed by the failure of all the specimens due to the excessive bending deflection due to the support of the concrete external steel tube.The yield of steel tube in the tensile zone is the turning point from elastic stage to elasticplastic stage,while the yield of steel tube in the compressive zone makes the specimen from elastic-plastic stage to plastic stage.Compared with concrete-filled steel tube members without internal stiffeners,the flexural capacity of inner lattice steel bone and longitudinal stiffener members increased by 16.4%,and the initial flexural stiffness increased by 14.9%.(2)The finite element software ABAQUS is used to carry on the fine nonlinear finite element analysis to the test specimen,the finite element simulation result is basically consistent with the test result,and the reliability of the finite element analysis is verified.The failure process of concrete-filled steel tubular specimens with internal lattice structure is as follows: cracking of concrete in tensile zone → local compressive strength of concrete in compressive zone → yielding of steel tube in tensile zone →yielding of steel bone in tensile zone,yielding of steel tube in compressive zone,yielding of steel bone in compressive zone → yielding of steel tube in compressive zone,yielding of large area of steel tube in compressive zone,excessive crack of concrete in tensile zone and extending of concrete in compressive zone from outside to inside large area to reach compressive strength and specimen failure.Compared with the development process of neutralization axis of each specimen,it can be seen that the internal lattice angle steel delays the rise of neutralization axis during loading,reduces the height of neutralization axis in each stage of the specimen,and makes more concrete participate in the sharing of pressure,thus improving the flexural bearing capacity of the specimen.(3)By using the finite element software ABAQUS the systematic parameter analysis is carried out to study the influence of the steel tube diameter to thickness ratio,steel bone size,steel limb spacing,steel bone steel strength,core concrete strength and other parameters on the flexural bearing performance of the members.The results show that the parameters which have great influence on the ultimate flexural bearing capacity of the specimen are the ratio of diameter to thickness of steel tube,the size of steel bone and the strength of steel.Considering the bending properties,it is suggested that the strength of steel reinforced concrete members should be of the same grade as steel tube or slightly lower than that of steel tube.A value of 0.50～0.54 can be obtained according to the ratio of the distance between steel bone and outer diameter of steel tube.(4)The flexural stiffness and flexural bearing capacity of concrete-filled steel tube members of different structural forms are calculated according to China’s Technical Code for Concrete-filled Steel Tubular Structures(GB 50936-2014),Design Specification for Specified Concrete-filled Steel Tubular Members(CECS 408:2015),United States Code(AISC-LRFD and AISC360-16)and European Code(Eurocode 4).Comparison with the test results shows that the calculated value of U.S.Code(AISC360-16)is the closest to the test value for the flexural stiffness of steel-filled steel tube concrete specimen with internal lattice structure.China’s Code for Design of Special Concrete-filled Steel Tube(CFST)Members CECS 408:2015 The results of calculation of flexural capacity of specimens with internal lattice steel bones are conservative.On this basis,combined with the results of test and finite element analysis,the calculation formula of flexural bearing capacity of inner steel reinforced concrete filled steel tubular members is put forward,and the calculation results of the formula are in good agreement with the test results and the finite element simulation results.